1. Field of the Invention
The present invention relates to a flow velocity detector for detecting flow velocity of a detected fluid (for example, air).
2. Description of the Related Art
Conventionally, as a flow velocity detector of this kind, a diaphragm type flow velocity detector consisting of two sensors and a heater element has been used. FIG. 13 is an approximate cross section structure of a conventionally existing diaphragm type flow velocity detector. In addition, FIG. 12 is a plan view of this diaphragm portion. In FIG. 12 and FIG. 13, 1 is a silicon chip (a substrate), 1-1 is a diaphragm portion being formed in a thin wall shape providing a cavity 1-2 on a top face of a substrate 1, 2 (2-1) is a heater (a heater) of a metal thin film formed on the diaphragm portion 1-1, 3U (3U0) and 3D (3D0) are heat-sensitive resistor elements (temperature sensors) of the metal thin film formed on both sides of the heater 2, and 4 is a slit penetrating through the diaphragm.
The heater 2 and the temperature sensors 3U, 3D are covered by an insulating layer 5 of thin film consisting of silicon nitride, for example. The heater 2 and the temperature sensors 3U, 3D are formed in a comb tooth-shape connecting a plurality of crank shapes, and a recessed and projected direction of this comb tooth-shape is positioned approximately perpendicularly to the flow direction A of a detected fluid.
A principle of a flow rate measurement method using this flow velocity detector is described. The heater 2 is driven so that its temperature becomes higher by a predetermined temperature than the ambient temperature, and the temperature sensors 3U, 3D are driven with a constant current or a constant voltage. Under the driving condition thus described, when a flow rate of a detected fluid is zero, the temperature of the temperature sensors 3U, 3D becomes the same, and no differences are caused in the resistance values of the temperature sensors 3U, 3D. When there is a flow of a detected fluid, the temperature sensor (an upstream side temperature sensor) 3U located upstream is cooled, because its heat is carried away by the flow of the detected fluid that goes to the direction of the heater 2. On the other hand, the temperature sensor (a downstream side temperature sensor) 3D located downstream is heated by the flow of the detected fluid heated with the heater 2. By the occurrence of the temperature difference between said upstream side and down stream side, the differences are caused in the resistance values of the upstream side temperature sensor 3U and the downstream side temperature sensor 3D. In the above-mentioned flow velocity detector, a flow rate of the detected fluid is obtained by detecting the difference of this resistance value as the difference of a voltage value.
However, there was a problem in the above-mentioned conventional flow velocity detector that, when a flow rate of the detected fluid becomes, for example, larger than 20 m/s, the temperature of the temperature sensors 3U, 3D becomes saturated making it impossible to detect a flow rate. The relationship between a flow rate and a sensor output is shown in FIG. 11. The characteristics I as shown in said drawing are the flow ratexe2x80x94sensor output characteristics of a conventional flow velocity detector, and the sensor output is saturated from the neighborhood of 20 m/s, making it impossible to detect a high flow rate. This is caused by the low heat association between the heater 2 and the temperature sensors 3U, 3D.
As the constitution to raise the heat association between the heater 2 and the temperature sensors 3U, 3D, the applicant of this invention has proposed a flow rate sensor as shown in the Official Gazettes of the Japanese Patent Publication No. Hei 4-74672 (prior application 1) and the Japanese Patent Publication No. Hei 6-68451 (prior application 2). In the prior application 1, the heat association is raised by superposing a heater and a temperature sensor on the diaphragm, making it possible to detect a high flow rate. In the prior application 2, the heat association is raised by covering with a metal layer the top of a heater and a temperature sensor which are placed beside on the diaphragm, making it possible to detect a high flow rate.
However, in the prior application 1 and the prior application 2, each had a problem that the manufacturing was difficult and the mass production was difficult also, although the detection of a high flow rate was made possible by raising the heat association between the heater and the temperature sensor.
For example, in prior application 1, a pattern of a heater is formed, in the first place, on the surface of a diaphragm by the first production process. Next, an insulation film is formed on the formed heater (the second production process), and further, a pattern of a temperature sensor is formed on an insulation film (the third production process). In this case, at the time of the completion of the first production process, when it is observed from the micro-viewpoint, it has a form that a pattern of a heater is protruded from the diaphragm surface. Generally, it is difficult to form a pattern by the subsequent production process on the surface of such an irregularity, and it is because of this that a mass production has become difficult. In addition, it is difficult to secure the reliability of the electrical insulation between a heater and a temperature sensor by the above-mentioned second production process, making it difficult to proceed with the mass production.
In addition, there is the situation that the mechanical distortion will be caused by a difference of a thermal expansion of each film, when the temperature sensor is formed on the heater as in the prior application 1, and when the top of the heater and the temperature sensor is covered with a metal film layer as in the prior application 2. Said mechanical distortion influences the resistance value of the temperature sensor (temperature sensor acts like a distortion gauge), causing an error of flow rate detection value to become greater, and in particular, a characteristics dispersion in the product at the time of a mass production increases, and as a result, the mass production becomes difficult.
In order to eliminate the above-mentioned problems, in the flow velocity detector according to the present invention, a temperature sensor is disposed with the recessed and projected direction of a comb tooth-shape positioned approximately in parallel with a flow direction of a detected fluid.
According to this invention, the heat from a heater heats the crank-shaped connection parts of the temperature sensors formed in a comb tooth-shape, and then transfers through straight line parts along the crank shapes of the temperature sensors in the direction approximately in parallel with the flow direction of the detected fluid.